Slow Scanning Speed Research Articles

Thermal features and its effect on the properties of AISI 4140 fabricated by conventional and extreme high-speed laser material deposition

The extreme high-speed laser material deposition (EHLA) process has the potential to enable additive manufacturing for mass production by overcoming the limitations of slow scanning speed in conventional laser material deposition (LMD) processes. Thermal features are the key factors to link process and properties. A sound understanding of process-thermal-property relationships is essential for performance control and process optimization of a deposited component. In this study, we studied the thermal characteristics within a single layer and among layers for continuous processing using the numerical method, and reveal the mechanism of microstructure and hardness changes of AISI 4140 material formed by EHLA and LMD processes through the analysis of thermal properties and temperature history results. The grain size and hardness evolution for both processes during single-layer cladding and continuous forming processes were investigated. The results revealed that the grain refinement effect in continuous LMD processing is stronger than that in EHLA process (from 30.0 μm for single layer to 3.2 μm for multi layers vs. from 18.6 μm for single layer to 2.2 μm for multi layers). Similar hardness values were obtained by LMD and EHLA processes, with mean values of 511 HV and 472 HV, respectively. The yield and tensile strengths of EHLA were superior to the conventional cast material, but inferior to those of the conventional material quenched and tempered at lower temperatures. The enhanced tensile results of EHLA process were found similar to those prepared through conventional method with quench and tempering at 600 °C.

Effect of laser speed on neck formation in SS316L powder joining for LPBF

The objective of this study is to comprehend how changes in laser speed affect the behaviour and features of neck formation in SS316L during powder bed fusion. This study examines the impact of various laser speeds on the neck formation in the commonly used SS316L. Uniweld Laser 3000 was used for laser joining. A constant laser power of 15 W and laser scan speed of 25–150 mm/s were implemented, and powder joining within a single track was observed. Results show that neck size decreases with increasing laser scanning speed and no particle joining occurs at 150 mm/s scan speed, as it is considered too fast for sufficient heat absorption and melting. As laser speed decreases to 125 mm/s, neck starts to form because the particles absorb more heat, which facilitates melting and bonding. Surface diffusion occurs early at this stage because it requires only a small amount of energy to break atomic bonds on the surfaces of particles compared to those in the internal part. This process involves atoms moving along the surfaces of the particles, facilitating the initial stages of sintering by reducing surface energy and causing neck formation between particles. At 125 mm/s, the particle shape remains spherical, and the average neck size is 49 μm. At 100 μm, the powder average neck size is 62 μm. Moreover, the powder completely merges at 50 mm/s and 25 mm/s. The neck formed between particles considerably improves the product's mechanical strength by strengthening bonds between particles. Material density increases with neck quality, and stable neck growth can reduce the porosity of the final parts. Ensuring thermal stability in the laser powder bed fusion allows consistent heat to particles, ensuring even temperature in materials and ultimately facilitating stable neck growth. Thermal stability enables atoms in particles to migrate efficiently and prevents defects, such as bubbling, which increases porosity in printed parts. Therefore, a slow scanning speed allows particles to absorb more heat for melting, increasing the neck size of particles. This knowledge is useful for optimizing laser processing parameters in various industrial applications.

Progressive Induction Hardening: Measurement and Alteration of Residual Stresses

Progressive induction hardening is an in-line steel heat treatment method commonly used to surface harden powertrain components. It produces a martensitic case layer with a sharp transition zone to the base material. This rapid process will induce large residual stresses, where a compressive state in the case layer will shift to a tensile state in the transition zone. For fatigue performance, it is important to quantify the magnitude and distribution of these stresses, and moreover how they depend on material and processing parameters. In this work, x-ray diffraction in combination with a layer removal method is used for efficient and robust quantification of the subsurface stress state, which combines electropolishing with either turning or milling. Characterization is done on C45E steel samples that were progressively induction hardened using either a fast or slow (27.5 or 5 mm/s, respectively) scanning speed. The results show that although the hardening procedures will meet arbitrary requirements on surface hardness, case depth and microstructure, the subsurface tensile stress peak magnitude is doubled when using a fast scanning speed. However, the near-surface compressive residual stresses are comparable. In addition, the subsurface tensile residual stress peak is compared with the on-surface tensile stresses in the fade-out zone.

In-fiber illumination-detection-synchronized confocal microscopy for high-precision fast axial scanning

Confocal microscopy remains a major workhorse in the 3D surface topography measurement owing to its reliability and compatibility for workpieces with various reflectance, which, however, suffers from slow scanning speed and limited axial resolution. Commercial confocal microscopy primarily relies on the axial movement of a sample or objective lens to create a series of 2D image stacks, and then reconstruct 3D map. Nevertheless, most samples and objective lens are too bulky to scan rapidly, let alone that multiple scans are required to increase the signal-to-noise ratio (SNR). To address this issue, an original approach named illumination-detection-synchronized confocal microscopy was developed, which enabled high-precision fast axial scanning via a compact fiber-multiplexed port with the high-speed synchronous motion of point source and point detection. The approach can effectively improve the peak localization precision while not requiring a high-precision motion stage. Theoretically, via controlling the magnification of the optical system M > 1, the total peak localization precision of axial light intensity response curve can be improved by M3 times. Experiments show that the single-point repeat measurement precision by this approach is improved by 4 times, and the profile measurement precision is enhanced by 2.3 times. Moreover, this approach can effectively avoid collisions between the sample and objective lens, and has the potential to design a low-cost single-point confocal sensor or confocal profilometer.

تأثير مدخلات الليزر Nd-YAG)) على الصلادة لصلب العدة عند درجات مختلفة من الخشونة السطحية

This research study focuses on the experimental analysis of surface hardening with laser rays for a sample of tool steel 1.2379 at different surface roughness's and laser parameters.The sample was received in an annealed condition with hardness value of 220 HV. The purpose of the experiment was to determine the effect of surface roughness and laser scanning speed on the surface hardness of the sample and the depth of the molten layer. The results showed that the surface roughness and laser scanning speed influenced the depth and hardness of the provided surface. It was observed that at slower scanning speed and a higher surface roughness, a higher hardness and a deeper cured layer thickness were achieved. A maximum surface hardness of 662 HV was achieved using a laser power of 75 W, a laser spot diameter of 0.5 mm, a pulse rate of 80 kHz, a laser processing speed of 1 mm/s, and at the higher surface roughness. Keywords: Tool Steel, Laser Surface Hardening, Surface Roughness, Microhardness, Scanning Speed.

Investigation of effects of process parameters on microstructure and fracture toughness of SLM CoCrFeMnNi

Selective Laser Melting (SLM) of multi-component alloy (MCA) such as CoCrFeMnNi is gaining significant attention due to its potential to exhibit an intriguing blend of mechanical properties, particularly strength-ductility combinations. However, limited attention has been directed toward the critical aspect of fracture toughness, which is indispensable for various engineering applications. This study delves into obtaining the fracture toughness of SLMed CoCrFeMnNi MCA, with a focus on the influence of microstructure, densification, and key printing parameters (volumetric energy density (VED), laser power, and scan speed). Experimental findings revealed that individual printing parameters rather portray better insights on the densification as opposed to the VED. Notably, slow scan speed of 800 mm/s significantly enhances densification irrespective of the VED. The alloy presents complex cellular structures with the morphology reflecting a non-linear dependence on VED. The presence of cellular dislocations analogous in scale to the cellular structures was observed with dark nanoprecipitates, recognized as Mn-rich oxides. Microhardness evaluations portrayed a non-linear relationship with varying VED, with values spanning between 230 HV and 280 HV. The fracture toughness did not exhibit a linear correlation also with VED, but better fracture toughness values were achieved at 200 W. Additionally, a notable observation of late crack initiation was observed on the load-displacement curves, indicative of the material's intrinsic ductility. The fracture toughness values ranged between 320 and 220 MPa.m−0.5. A correlation between the microstructure and densification with the fracture toughness was made. The paper also presents a comprehensive discussion on the mechanisms underpinning microstructure formation and their consequential effects on the fracture toughness of the material.

Surface Roughness of Interior Fine Flow Channels in Selective Laser Melted Ti-6Al-4V Alloy Components.

A challenge remains in achieving adequate surface roughness of SLM fabricated interior channels, which is crucial for fuel delivery in the space industry. This study investigated the surface roughness of interior fine flow channels (1 mm diameter) embedded in SLM fabricated TC4 alloy space components. A machine learning approach identified layer thickness as a significant factor affecting interior channel surface roughness, with an importance score of 1.184, followed by scan speed and laser power with scores of 0.758 and 0.512, respectively. The roughness resulted from thin layer thickness of 20 µm, predominantly formed through powder adherence, while from thicker layer of 50 µm, the roughness was mainly due to the stair step effect. Slow scan speeds increased melt pools solidification time at roof overhangs, causing molten metal to sag under gravity. Higher laser power increased melt pools temperature and led to dross formation at roof overhangs. Smaller hatch spaces increased roughness due to overlapping of melt tracks, while larger hatch spaces reduced surface roughness but led to decreased part density. The surface roughness was recorded at 34 µm for roof areas and 26.15 µm for floor areas. These findings contribute to potential adoption of TC4 alloy components in the space industry.

Validation of widefield aberrometer for global peripheral aberration scanning

Aberrometers based on Hartmann-Shack wavefront sensing methods have been widely used in the assessment of aberrations in the human eye, a crucial aspect in addressing or adjusting peripheral vision for myopia control, among other applications. Nevertheless, most available aberrometers exhibit limitations such as slow scanning speeds and restricted visual fields when measuring peripheral aberrations. Even the fastest scanning capabilities of commercialized aberrometer are confined to limited visual field and meridians. In this paper, a novel scanning aberrometer for efficient measurement of human eye peripheral aberrations is developed. This instrument allows the measurement of a large visual field angle up to ± 50° and takes all meridian scanning into account, which means it is a widefield aberrometer that can scan the peripheral aberrations globally. Due to the use of the precision motorized rotation stage, both the meridional and visual field angular resolutions of this instrument can be lower to 1°. During the entire measurement process, the subject only needs to focus on a single fixation target. No eye turn or head turn is required. By the instrument installation completed in April 2023, it stands as the only peripheral aberration scanning aberrometer capable of measuring up to a 50° visual field at all meridians using a single fixation target and achieving high angular resolution.

High-Speed AFM Observation of Electrolytic Nanobubbles Formation on HOPG

IntroductionNanobubbles (NBs) on solid surfaces are an interesting topic for surface and colloid science. Since the diameter of NBs is less than 1 μm, NBs can be morphologically measured by atomic force microscope (AFM) [1]. However, due to the slow scanning speed of conventional atomic force microscope (AFM), the nucleation and growth process is not yet known. We previously applied high-speed AFM (HS-AFM) for NBs’ evolution during water electrolysis [2]. Here, we discuss the NBs’ formation during cyclic voltammetry (CV).ExperimentalThe experimental cell was a three-electrode system at room temperature. The working electrode was highly ordered pyrolytic graphite (HOPG). The counter and reference electrodes were Pt wires. The electrolyte was a sulfuric acid solution (5 mmol L-1). CV was performed by a potentiostat. HS-AFM was operated in dynamic mode. The scanning speed was 0.5 frame s-1 and the size was 1 × 0.75 µm2.Results and discussionWhen the potential was swept to the anode from the open circuit potential, the pancake-like NBs were formed (Fig. 1). The height was ~5 nm, and the surface roughness was very small (< 1 nm). The pancake NBs grew along the step side and moved actively on the surface. They occasionally coalesced with neighboring NBs.On the other hand, when the potential was swept to the cathode, the dynamic process was the opposite. The pancake NBs dissolved. This poster will present the dynamic variation of electrolytic NBs during the CV.Reference[1] X.H. Zhang et al., Langmuir, 23, 1778 (2007).[2] M Mita, H Matsushima, M Ueda, H Ito, J. Colloid Interface Sci., 641, 389 (2022). Figure 1

Microstructure and mechanical properties of 316L stainless steel of selected laser sintering

In recent years, additive manufacturing (AM) has received global attention, because the AM can manufacture irregular and complex-shaped parts and reduce material waste. In this paper, the 316L stainless steel powder was made into a structure conforming to the size of ASTM E8 by the selected laser sintering (SLS). The AM parameters were set as fixed laser power (220 W) and layer thickness of 0.03 mm, as well as scanning speed (900 mm/s and 800 mm/s), different annealing temperatures (750°C, 850°C and 950°C), and time (3 minutes, 6 minutes and 9 minutes). The results show that the optimal tensile strength was 639 MPa when the scanning speed was 800 mm/s, and the annealing temperature and time were 750°C and 3 minutes, respectively. In addition, the maximum elongation was 43% when the annealing temperature and time were 850°C for 6 minutes and 9 minutes, and 950°C for 9 minutes. The as-SLS of fast and slow scanning speeds were compared, the maximum tensile strength increased from 575 MPa to 639 MPa (increased by 11.1%), and the elongation increased from 21% to 32% (increased by 13.1%). The significant records of the 316L mechanical properties are extremely important for the fundamental understanding of SLS.

Improved Leakage Detection and Recognition Algorithm for Residual Neural Networks Based on Transfer Learning

Due to the lack of other component information in traditional magnetic leakage signal defects and the low accuracy of prediction methods, this paper proposes an improved residual network for magnetic leakage detection defect recognition method that predicts defect size and different detection speeds. A new defect diagnosis method based on ResNet18 on the Convolutional Neural Network (CNN) is proposed in this study. This method transfers the pre-trained ResNet18 network and replaces the activation function in the transferred network structure. It extracts features from transformed two-dimensional images obtained by converting the original experimental signals and signals with added noise, removing the influence of manual features. The results demonstrated that the improved ResNet18 network model, after transfer learning, achieved 100% prediction accuracy for all 10,000 grayscale images generated with defect lengths of 50 mm; width of 2 mm; and depths of 2 mm, 4 mm, 6 mm, and 8 mm. Moreover, the prediction accuracies for the quasi-static, slow, compensated fast, and fast scanning speeds were 99.20%, 98.50%, 93.30%, and 94.00%, respectively, for defect depths of 2 mm, 4 mm, 6 mm, and 8 mm. These accuracies surpass those of other models, demonstrating the significant improvement in prediction accuracy achieved by this method.

Picosecond pulsed 532nm laser system for roughening and secondary electron yield reduction of inner surfaces of up to 15m long tubes.

Laser-induced surface structuring is a promising method to suppress electron mulitpacting in the vacuum pipes of particle accelerators. Electrons are scattered inside the rough surface structure, resulting in a low Secondary Electron Yield (SEY) of the material. However, laser processing of internal pipe surfaces with a large aspect ratio is technologically challenging in terms of laser beam guidance and focusing. We present a 532nm ultrashort-pulse laser setup to process the inner parts of 15m long beam vacuum tubes of the Large Hadron Collider (LHC). Picosecond pulses at a repetition rate of 200kHz are guided through an optical fiber toward an inchworm robot traveling inside the beam pipe. The system was installed, characterized, and tested for reliability. First surface treatments achieved the required scan precision. Cu2O-dominated nano-features were observed when processing at high average laser power (5 W) and slow scanning speed (5mms-1) in nitrogen flow, and the maximum SEY of copper was decreased from 2.1 to 0.7.

An edge-preserving total nuclear variation minimization algorithm in EPR image reconstruction

ObjectiveCurrently, slow scanning speed has become a technical bottleneck limiting the development of electron paramagnetic resonance imaging (EPRI), a novel oxygen imaging modality. Image reconstruction from sparse-view projections is an important imaging configuration to achieve fast scanning in EPRI. However, EPR images reconstructed from sparse-view projections using traditional analytic algorithms suffer from sparse artifacts, causing severe image degradation. In this work, we aim to use an optimization-based approach to achieve high-precision sparse reconstruction for EPRI. Methods. Inspired by the success of total nuclear variation (TVN) for denoising multi-channel spectral CT images and adaptive-weighted total variation (awTV) for improving the edge-preserving performance of TV, we proposed an edge-preserving total nuclear variation (EPTVN) minimization algorithm for sparse reconstruction in EPRI. The EPTVN combines the advantage of TVN in modelling structural similarity and low-rank prior between multi-channel images for denoising with the advantage of awTV for edge-preserving, which can better protect the edge structure of images while suppressing sparse artifacts. Results. We designed two numerical phantoms with different properties, a piecewise-constant phantom and a gradient phantom, and a physical phantom, and then performed inverse crime, simulation study and real-data study. Experimental results show that the EPTVN minimization algorithm can reconstruct more accurate EPR images from sparse-view projections for both the numerical and physical phantoms. Significance. The proposed method can effectively suppress sparse artifacts and protect edge structures of reconstructed images, thus achieving high-precision sparse-view reconstruction for EPRI. The insights of this work can also be applied to multi-channel image denoising tasks.

(Digital Presentation) Quantifying the Effect of Potential Cycling Conditions on the Resulting Performance of Stainless Steel as an Anode for Alkaline Water Electrolysis

Alkaline water electrolysis through the use of the anion exchange membrane (AEM) has great potential. The current industry standard for water electrolysis utilise diaphragm-based traditional alkaline water electrolysers (AWEs), however using AEM water electrolysers (AEMWEs) introduces benefits including greater current densities, superior hydrogen purity, simpler cell/stack designs and less corrosive electrolytes. These advantages are additions to the pre-existing benefits to AWE, namely the possibility of utilising inexpensive catalyst materials for both hydrogen and oxygen evolution.Great attention has been payed to stainless steel (SS) as an anode for AEMWE due to its fair activity, low cost and good stability. Potential cycling (PC) is one method of electrochemically modifying the surface of various SS structures to increase electrochemical activity, however the PC conditions thus far reported in literature are rife with variation. As such, the full extent of PC conditions on SS remain unreported. Herein, we seek to fill this gap in literature by potential cycling a series of SS felt (SSF) electrodes under varied scan rates and ranges. Special attention is payed to surface conditions due to the intricate nuances affecting the electrocatalytic activity of the surface oxide layer.Two redox couples are clearly visible in cyclic voltammetry (CV) in the range 0-1.90 VRHE, termed whole-range, namely the Fe+2/Fe+3 at 0-0.65 VRHE and Ni+2/Ni+3 at 1.50-1.75 VRHE. The SSF electrodes were PC over one of these ranges at either slow, intermediate or fast scan rates, producing six different SSF electrodes.Initiating measurements were carried out to ascertain the baseline performance, including whole-range CV, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). The pristine SSF surface was exceedingly sensitive to the whole-range CV sweeps during initial measurements. Thus, this work is divided between SSF electrodes with and without CV in the initial measurements, termed with and without pre-cycling.Without pre-cycling, the decline in charge transfer resistance (Rct) before and after PC was more than tenfold for all scan speeds. Scan speed was relevant, where slow and fast scan speeds resulted in a mean Rct reduction of 84.15 and 90.59% for fast and slow scan speeds respectively. Post-experimental surface analysis with X-ray photoelectron spectroscopy revealed a decline in the average oxidation state of the principle elements in the SSF electrodes (Fe, Ni and Cr) and a surface depletion of iron. The relative increase in surface concentration of nickel and chromium was highly correlated with the reduction of Rct. Evaluation of affiliated Tafel slopes and LSV curves reveals similar trends, where the latter indicates a fair performance increment between 8-21% for fast and slow scan speeds.With pre-cycling the decline in Rct was lower, in the range of 18-30% and 3-7% for the SSF electrodes cycled around the Fe+2/Fe+3 and Ni+2/Ni+3 redox couple respectively. The influence of scan rate was the same for these SSF electrodes as with those unexposed to pre-cycling. Tafel analysis reveals two dominant slopes, where PC elicits small changes in the low current density slope and greater changes in the high current density region for the Fe+2/Fe+3 cycled SSF electrodes. These trends are also seen in LSV curves, where the greatest improvement is clearly seen for the SSF electrode cycled with a slow scan rate.Tafel analysis shows that the SSF electrodes cycled around the Ni+2/Ni+3 redox couple display a small decline in kinetics following PC, which corresponds to the rather meagre improvement in Rct affiliated with the PC conditions. The same trends are also seen by comparing the before and after LSV curves.Additional measurements documenting the full cell performance of these anodes is necessary and will be featured in the unabridged version of this paper, in addition to a more thorough characterisation of the surface conditions.

A fast image inpainting algorithm based on an adaptive scanning strategy

OBJECTIVES: In exemplar-based image inpainting algorithms, there are often issues with the calculation of patch similarity for matching, suboptimal strategies for selecting matching patches, and low inpainting speed.METHODS: This paper first uses the variable scale cross-scan block line progressive scan to solve the problem of slow scanning speed and invalid priority formula. Then, an improved weight similarity formula is used for searching to solve the problem of poor computing strategy for similar matching patches. The search range of matching patches gradually increases from small to large until globally searching for similar matching patches to improve the efficiency of inpainting. To further improve the correctness of matching patch selection, this paper uses six levels of priority matching criteria for screening.RESULTS: The experimental results show that the inpainting effect of the proposed method is significantly improved in subjective vision, and the structural similarity (SSIM), peak signal-to-noise ratio (PSNR), and inpainting speed of the inpainting results are all improved.CONCLUSION: For different types of images, the proposed method has a better inpainting effect and higher inpainting speed than the other three advanced methods.

High-speed, multi-zone ultrasonic inspection of bar and wire stocks with an in-line phased array inspection system

Typical standards in aerospace applications require the ultrasonic testing (UT) of bar and wire stock with a highly reliable detection of small defects over a wide depth range in the material. Characteristic reference reflectors are flat bottom holes (FBH) with diameters smaller than 1 mm. Typically, such inspections are done with immersion tanks using a set of probes optimized for each depth. However, for industrial applications there are disadvantages such as slow scan speed, the time-consuming inlet and outlet process, complicated mechanics and significant costs of the specialized probes. This contribution demonstrates an in-line phased array inspection system with integrated multi-zone inspection, which is based on the Krautkrämer ROWA™ Ux machine. After a short description of the setup special focus is given on the implemented testing concept which is optimized with respect to a multi-zone inspection and includes a dedicated automated adjustment of the system. Furthermore, the powerful electronics allows a flexible setting of focus point, aperture size and filtering. This significantly reduces the need for special probes and by means of digital depth focusing (DDF) also the number of test shots. At the end, the flaw detectability in different depths is illustrated and achievable test speeds are presented. Moreover, external boundary conditions are discussed.

Surface microstructure evolution of Cf/SiC ceramic matrix composite microgrooves processed by ultrafast laser

Modern aviation components have higher requirements for high temperature resistance, high strength and lightweight materials, and ceramic matrix composites have superior overall performance. However, its high brittleness and anisotropy lead to a challenge for manufacturing. In order to understand the formation conditions and the evolution of surface microstructures of the Cf/SiC microgrooves processed by ultrafast laser comprehensively, we designed a single-factor experiment and performed sensitivity analysis. The experiment results showed that the pulse energy had great effects on the depth of the microgroove, and the intense ablation caused more active oxidation of SiC to occur, generating more SiO(g). However, too much pulse energy may cause the material removal mechanism to be more due to the photothermal effect rather than the plasma effect. Low repetition frequency caused a large number of laminated connections in the microgroove and the oxide gradually changed from lumpy to flocculent as the repetition frequency increased. The more scanning times, the more ablation products sputtered onto the sample surface, including unablated carbon fibers. Shallow depth and ablation residues remained in the microgroove occurred under few scanning times. Although too fast scanning speed leaded to a rapid decrease in the microgroove depth, too slow scanning speed also generated more unablated carbon fibers sputtering out of the microgrooves. The microgroove depth had the highest sensitivity to the repetition frequency, followed by the pulse energy and scanning speed. The pulse energy and scanning speed had a greater effect on the oxide layer height, the repetition frequency affected the oxide layer width, and the scanning speed affected the microgroove width significantly. According to the processing requirements and the hot spot map, the processing parameters that can be adjusted effectively will be able to be obtained.

Numerical Simulations to Predict the Melt Pool Dynamics and Heat Transfer during Single-Track Laser Melting of Ni-Based Superalloy (CMSX-4)

Computational Fluid Dynamics (CFD) simulations are used in this work to study the dynamic behavior of the melt pool and heat transfer during the single-track laser melting process of a nickel-based superalloy (CMSX-4). To include the effects of powder inhomogeneities and obtain a realistic distribution of the powder layer on the bed chamber, the CFD model is coupled with a Discrete Element Method (DEM) solver. The coupled model is implemented in the open-source software package OpenFOAM. In the CFD model’s governing equations, some key physical mechanisms, such as the Marangoni effect and recoil pressure, are considered. With the help of the coupled CFD-DEM model, we have investigated the effect of key process parameters, such as laser power, scanning speed of the laser, powder size, and powder shape, on the size and homogeneity of the melt pool. From the simulation results, it was discovered that high laser power and slow scanning speed create a deep and narrow keyhole that leads to porosity. In contrast, balling defects are found to be caused by a small melt pool obtained from fast scanning speeds and inadequate laser power.

High-speed multiparametric imaging through off-resonance tapping AFM with active probe

Off-resonance tapping (ORT) mode of atomic force microscopy (AFM), based on force-distance curve, is widely concerned due to its advantages of weak tip-sample interaction and concurrent quantitative property mapping. However, the ORT-AFM still has the disadvantage of slow scan speed caused by low modulation frequency. In this paper, we overcome this disadvantage by introducing active probe method. With active probe, the cantilever was directly actuated with the induced strain after applying voltage in the piezoceramic film. In this way, the modulation frequency could be increased to more than an order of magnitude faster than that of traditional ORT, thus improving the scan rate. We demonstrated high-speed multiparametric imaging with the active probe method in ORT-AFM.

Study of Sparse Terahertz Synthetic Aperture Imaging Algorithm

For the terahertz synthetic aperture near-field imaging problem, this paper proposes a SAR imaging method based on sparse reconstruction, which solves the problems of slow scanning speed, a large amount of data, and complicated calculations in the terahertz synthetic aperture imaging process. The targets in space are sparsely sampled and reconstructed using a compressed sensing reconstruction method. Simulation results show that high-quality target reconstruction can be achieved at low sampling conditions.